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<Paper uid="P92-1030">
  <Title>TENSE TREES AS THE &amp;quot;FINE STRUCTURE&amp;quot; OF DISCOURSE</Title>
  <Section position="3" start_page="0" end_page="0" type="intro">
    <SectionTitle>
1 Introduction
</SectionTitle>
    <Paragraph position="0"> Work on discourse structure, e.g., \[Reichman, 1985; Grosz and Sidner, 1986; Allen, 1987\], has so far taken a rather coarse, high-level view of discourse, mostly treating sentences or sentence-like entities (&amp;quot;utterance units, .... contributions,&amp;quot; etc.) as the lowest-level discourse elements. To the extent that sentences are analyzed at all, they are simply viewed as carriers of certain features relevant to supra-sentential discourse structure: cue words, tense, time adverbials, aspectual class, intonational cues, and others. These features are presumed to be extractable in some straightforward fashion and provide the inputs to a higher-level discourse segment analyzer.</Paragraph>
    <Paragraph position="1"> However, sentences (or their logical forms) are not in general &amp;quot;flat,&amp;quot; with a single level of structure and features, but may contain multiple levels of clausal and adverbial embedding. This substructure can give rise to arbitrarily complex relations among the contributions made by the parts, such as temporal and discourse relations among subordinate clausal constituents and events or states of affairs they evoke. It is therefore essential, in a comprehensive analysis of discourse structure, that these intra-sentential relations be systematically brought to light and integrated with larger-scale discourse structures.</Paragraph>
    <Paragraph position="2"> Our particular interest is in tense, aspect and other indicators of temporal structure. We are developing a uniform, compositional approach to interpretation in which a parse tree leads directly (in rule-to-rule fashion) to a preliminary, indezical logical form, and this LF is deindezed by processing it in the current context (a well-defined structure). Deindexing simultaneously transforms the LF and the context: context-dependent constituents of the LF, such as operators past, pres and perf and adverbs like today or earlier, are replaced by explicit relations among quantified episodes; (anaphora are also deindexed, but this is not discussed here); and new structural components and episode tokens (and other information) are added to the context. This dual transformation is accomplished by simple recursive equivalences and equalities. The relevant context structures are called tense trees; these are what we propose as the &amp;quot;fine structure&amp;quot; of discourse, or at least as a key component of that fine structure.</Paragraph>
    <Paragraph position="3"> In this paper, we first review Reichenbach's influential work on tense and aspect. Then we describe temporal deindexing using tense trees, and extensions of the mechanism to handle discourse involving shifts in temporal perspective.</Paragraph>
  </Section>
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